This type of turbomachine is used in particular in the form of an expansion turbine or a high pressure pump in the liquefied natural gas (LNG) and the liquefied petroleum gas (LPG) industries. The fluid passing through the turbomachine is then natural gas (methane, ethane), liquefied petroleum gas (butane, propane), or a mixture thereof referred to as a refrigerant-mix (RM) (which may also include a limited quantity of heavier compounds).
A permanent objective when designing such a machine is to maximize its lifetime and performance.
For that purpose, efforts are made in particular to reduce the rotary friction of the rotor. To do this, it is essential to select appropriate means for holding the position of the rotor relative to the stator, both radially and axially. It should be observed that usually the rotor is arranged with a shaft that is disposed vertically, and a first function of the holding means is specifically to support the weight of the rotor, in particular when stationary or at low speed.
Various embodiments of holding means suitable for use in a turbomachine are disclosed in patent application US 2006/0186671.
In a first embodiment (FIG. 1), there is disclosed a turbine generator, with its radial position being held by rolling bearings. Nevertheless, ball bearings present the drawback of involving mechanical contact and thus friction between facing surfaces, thereby leading to wear and consuming energy, in particular at high speeds of rotation; they also put a limit on the maximum acceptable speed of rotation.
Axial position is held at low speed by one of the above-mentioned ball bearings, and at high speed by an axial balancing device arranged on the back of a bladed wheel.
The above-mentioned ball bearing includes an outer ring mounted slidably relative to the stator over a limited range. At low speed, the outer ring is blocked in abutment at one end of that range, with the ball bearing thus blocking the axial position of the rotor.
At high speed, the rotor is pushed upwards by the pressure of the fluid on the bladed wheels, and the axial balancing device takes over from the ball bearing for holding the rotor in a substantially stationary position, within the above-mentioned range. The axial balancing device is incorporated in the first bladed wheel and it includes a balancing chamber disposed on the rear face of the bladed wheel, associated with a nozzle presenting axial clearance and serving to adjust the pressure in the balancing chamber so as to hold the axial position of the rotor.
The drawback of that embodiment is that the inner ring of the ball bearing (secured to the rotor) and the balls are driven to rotate by the movement of the rotor, thereby giving rise to friction and wear in the ball bearing. As a result, the lifetime of that portion of the electricity generator is limited.
To remedy those drawbacks, patent application US 2006/0186671 presents another embodiment of a turbine electricity generator (FIG. 7).
In that embodiment, in order to hold the rotor radially, the generator has declutchable ball bearings. Those ball bearings are used at low speed. They are in parallel with hydrostatic bearings that take over at high speed. The hydrostatic bearings make use of the pressure of the fluid in the main circuit of the turbomachine and they hold the radial position of the rotor as soon as the speed of rotation (and thus the fluid pressure) reaches a sufficient value. (In that document, the “main circuit” of the turbomachine means all of the circuits enabling fluid to flow through the fluid-flow portion of the turbomachine.)
In order to hold the axial position of the rotor, when stationary and at low speed, one of the ball bearings is in abutment on a conical bearing surface of the rotor shaft, thereby supporting it in the vertical direction.
At higher speed, the rotor rises a little under the effect of the fluid pressure in the bladed wheels; it is then held in position by the axial balancing device. The ball bearings are then declutched by the rotor moving in translation, thereby opening clearance relative to the conical bearing surfaces of the rotor shaft in register with the ball bearings: the bottom bearing is declutched by the rotor moving in translation (separation from the cone), whereas for the top bearing, it is the rise in fluid pressure that acts against the device incorporating the bearing in order to declutch the cone.
That arrangement has the drawback of being complicated, and above all of lacking reliability. The ball bearings are declutched because the rotor moves in translation, and this movement is of very small amplitude. In practice, since there can be no question of using a shaft of large diameter, the conicity of the bearing surface remains small, thereby giving rise to poor reliability during the declutching of the bearings and running the risk of strong interaction in operation between the ball bearings and the hydrostatic bearings.
In addition, having parallel radial holding means (ball bearings in parallel with hydrostatic bearings) leads to extra weight and to an increase in the volume of the machine.